1. Field of the Invention
The invention relates to methods and apparatus for controlling the operation of xe2x80x9clean-burnxe2x80x9d internal combustion engines used in motor vehicles to obtain improved engine and/or vehicle performance, such as improved vehicle fuel economy or reduced overall vehicle emissions.
2. Background Art
The exhaust gas generated by a typical internal combustion engine, as may be found in motor vehicles, includes a variety of constituent gases, including hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx) and oxygen (O2). The respective rates at which an engine generates these constituent gases are typically dependent upon a variety of factors, including such operating parameters as air-fuel ratio (8), engine speed and load, engine temperature, ambient humidity, ignition timing (xe2x80x9csparkxe2x80x9d), and percentage exhaust gas recirculation (xe2x80x9cEGRxe2x80x9d). The prior art often maps values for instantaneous engine-generated or xe2x80x9cfeedgasxe2x80x9d constituents, such as HC, CO and NOx, based, for example, on detected values for instantaneous engine speed and engine load.
To limit the amount of feedgas constituents that are exhausted through the vehicle""s tailpipe to the atmosphere as xe2x80x9cemissions,xe2x80x9d motor vehicles typically include an exhaust purification system having an upstream and a downstream three-way catalyst. The downstream three-way catalyst is often referred to as a NOx xe2x80x9ctrapxe2x80x9d. Both the upstream and downstream catalyst store NOx when the exhaust gases are xe2x80x9cleanxe2x80x9d of stoichiometry and releases previously-stored NOx for reduction to harmless gases when the exhaust gases are xe2x80x9crichxe2x80x9d of stoichiometry.
Under one prior art approach, the current NOx-storing capacity of the trap is used as a predictor of the trap""s emissions-reducing performance and, preferably, lean engine operation is conditioned upon the trap exhibiting a minimum instantaneous NOx-storage capacity, perhaps as inferred from a measured instantaneous capacity to store oxygen.
And, because the trap""s NOx-storage capacity is known to decline in a generally-reversible manner over time due to sulfur poisoning or xe2x80x9csulfurization,xe2x80x9d as well as in a generally-irreversible manner over time due, for example, to component xe2x80x9cagingxe2x80x9d from thermal effects and xe2x80x9cdeep-diffusionxe2x80x9d/xe2x80x9cpermanentxe2x80x9d sulfurization, the prior art teaches the periodic scheduling of trap decontamination events, such as trap desulfurization events, designed to restore lost trap capacity. During one known desulfurization event, the temperature of the trap is raised to a relatively-elevated level, and a slightly-rich air-fuel mixture is provided for a relatively-extended period of time to release much of the stored sulfur and, hence, restore a portion of the trap""s lost capacity.
Unfortunately, as a further impact of trap sulfurization, empirical data suggests that a trap""s instantaneous NOx-storage efficiency, i.e., its instantaneous ability to incrementally store NOx, is increasingly affected by trap sulfurization as the trap begins to fill with NOx. Specifically, while a trap""s instantaneous efficiency immediately after a trap purge event is believed to remain generally unaffected by trap sulfurization, the instantaneous efficiency begins to fall more quickly, and earlier in the fill event, with increasing trap sulfurization. Thus, under certain circumstances, a relatively-low instantaneous trap efficiency may result in higher tailpipe emissions, even though the trap appears to have available NOx-storing capacity.
It is an object of the invention to provide a method and apparatus for qualitatively measuring the performance of a emissions control device.
In accordance with the invention, a method is provided for assessing the performance of an emissions control device coupled to a lean-burn internal combustion engine, wherein the device is operative to releasably store a quantity of a constituent of exhaust gas generated by the engine combustion engine when the engine is operating lean of stoichiometry and to release a previously-stored amount of the exhaust gas constituent when the engine is operating rich of stoichiometry. The method includes, during lean-burn operation, operating the engine at a first, rich engine operating condition to release substantially all previously-stored exhaust gas constituent from the device and then at a second, lean operating condition to store exhaust gas constituent in the device. The method also includes determining a first measure representative of an amount of the exhaust gas constituent entering the device when the engine is operating at the second operating condition, for example, by estimating an amount of the exhaust gas constituent generated by the engine when operating at the first operating condition based upon at least one of engine speed and engine load. The method also includes determining a second measure representative of an amount of the exhaust gas constituent exiting the device when the engine is operating at the second operating condition, for example, based upon a detected concentration of the exhaust gas constituent in the exhaust gas exiting the device. The method further includes determining a third measure based at least in part upon the first and second measures, for example, an efficiency measure determined as the difference between the first measure and the second measure, divided by the first measure.
In accordance with a feature of the invention, an exemplary method also includes initiating a third, rich engine operating condition when the third measure, preferably filtered over a plurality of successive device xe2x80x9cpurge/fillxe2x80x9d cycles, falls below a threshold value. In this manner, the invention advantageously ensures that lean-burn operation of the engine remains compliant with vehicle emissions standards with respect to the exhaust gas constituent.
Under the invention, a controller is similarly provided for controlling a lean-burn engine operating in combination with an emissions control device, wherein the device is operative to releasably store a quantity of a constituent of exhaust gas generated by the engine combustion engine when the engine is operating lean of stoichiometry and to release a previously-stored amount of the exhaust gas constituent when the engine is operating rich of stoichiometry. The controller is arranged to determine a first measure representative of an amount of the exhaust gas constituent entering the device when the engine is operating at the second operating condition, to determine a second measure representative of an amount of the exhaust gas constituent exiting the device when the engine is operating at the second operating condition, and to determine a third measure based at least in part upon the first and second measures.